Circuits that mix multiple input signals together to form a single output are well known in, for instance, the recording industry. Such circuits typically allow the gain of each input signal to be selected independently of any other input signal. The disadvantage of mixing circuits having such flexibility is that they require a significant number of components. In particular, such a circuit typically requires one operational amplifier for each signal channel being mixed as well as a separate operational amplifier for mixing the signals.
In consumer electronics there are products, such as cell phones, that have multiple audio input sources, including multiple microphone inputs, MP3 inputs and synthesizer inputs for ring tones. It is sometimes desirable to mix two or more of these audio sources into a single output channel, such as a headset phone. For instance, two people conversing by phone may also choose to listen to background music.
The mixing requirements of such consumer electronics mixing circuits are not as stringent as for the music industry as consumer electronics devices typically only require a limited number of circuit selection and mixing capabilities. The first capability that is typically required is the ability to select a channel and to adjust the gain of a signal in that selected channel. The second typically required capability is the ability to select a combination of channels, to mix them into a single output and to apply the same gain to all the signals from the selected channels. The third typically required capability is the ability, when a number of channels are selected and the signals from them mixed, to select a further subset of one or more channels and to apply a uniform gain to the signals from the selected subset while the gain of the signals from the remaining channels remains fixed.
Consumer electronic devices are, however, very price sensitive, making it highly desirable to provide such mixing capability using as few components as possible.
What is therefore desirable are low cost circuits, capable of providing the basic mixing capabilities typically needed by consumer electronics devices. One way of ensuring that the circuits are available at a low cost is to design them using as small a number of electronic components as possible, with particular attention to reducing the number of complex components such as operational amplifiers.